Profile duplicating machine



April 18 1944. EC. s. CLENCH EIAL PROFILE DUPLICATING MACHINE 2 She etsS-hee4t 1 Ad wad, f M Ii.

April 18, 1944. E. c. s. LE-H Em 2,346 823 PROFILE DUPLICATING MACHINE Filed March} 13, 1942 2 Sheets-Sheet 2 1! w I 51 56 53 l 111 0 .107 100 109 fare 7 aymgzd GHQ? Patented Apr. 18, 1944 2,346,823 PROFILE DUPLICATING MACHINE Edward Claude Shakespeare Clench and Quintin Healey Carlton, Leamington Spa, England, assignors to Automotive Products Company Limited, Leamington Spa, England Application March 13, 1942, Serial No. 434,590 In Great Britain March 17,1941

'1' Claims. (01. 90-135) This invention relates to profile duplicating machines of the kind (herein mentioned as the kind referred to) in which the work piece and the pattern or model are mounted so as to be moved in unison relative to a tool and a follower respectively, by means of a double-acting hydraulic motor unit having a pair of working spaces fed with pressure liquid from a supply system through the medium of a valve, which is operated by the follower and which is herein termed the follower valve. It will be appreciated that the tool and follower may be fixed, or alternatively the work piece and pattern may be fixed.

It is the object of the present invention to provide an improved liquid pressure system for r moving the table or the cutting tool in sympathy with movements which are imparted to the follower by the pattern or model, said system being particularly responsive and capable of producing accurate work.

In a profile duplicating machine of the kind referred to, according to the present invention the follower valve controls the flow of pressure liquid to and from only one of the working spaces of the motor unit, and a fluid actuated relay valve which is responsive to pressure changes in said one working space changes the pressure in the other working space of the motor unit, but in the sense opposite to that of the change of pressure in the said one space.

According to a further aspect of the invention,

in a profile duplicating machine of the kind retion a profile duplicating machine of the kind referred to, in which the follower valve directly controls the flow of pressure liquid into that working space of the motor unit where it urges the work and the tool into co-operative engagement, and liquid pressure in this working space actuates a pressure-sensitive relay valve, which itself controls the liquid pressure in the other working space, the pressure in said other working space being arranged to decrease when that in the said one working space increases, and vice versa.

, Preferably the relay valve comprises apiston valve member whichis arranged to connect the said other working space of the motor unit either with the pressure .liquidsupply or with a reservoir, said piston valve member being urged in one direction by a spring, and in the opposite direction by the pressure of the liquid in the said other working space of the motorunit, the said spring conveniently being arranged to urge the piston valve member towards the position; in which it connects the said other working space with the reesrvoir. If desired the piston valve member of the relay valve may be urged in opposition to the spring by the fluid pressure which is present in the said other working space. This piston valve member can be urged in opposition to the spring by the pressure of the working liquid ineach of the working spaces of the motor unit; thus the piston valve member of the relay valve may be acted upon by a plurality of plunger members, at least one of which is acted upon by the liquid pressure in one working space of the motor unit, and at least another is acted upon independently by the liquid pressure in the other working space thereof. z

The motor unit may comprise a cylinder which is arranged with its axissubs'tantially vertical, and which contains a piston, withworking spaces below and above said piston, the liquidpressure in the said one working space disposed below the piston being arranged to support the weight of the work and the work-carrying means. The invention is however also applicable where the cylinder axis is disposed other than vertically.

If desired the relay valve may comprise a body having coaxial main and auxiliary cylindrical chambers, the former of which contains a piston valve member, and the latter a piston with a thrust rod extending through a partition between the chambers, a pair of relatively strong coiled compression springs being disposed within the respective chambers with the thrust rod operatively connecting them so that they both urge the piston valve member towards its on position against the opposing influence of the pressure in the said other working space of the motor unit, while the liquid in the said one working space of the motor unit is connected with the auxiliary chamber of the relay valve so as to oppose the force imparted to the thrust rod by the spring within said auxiliary chamber. In a somewhat similar manner the follower valve can comprise a body having a bore with three axially spaced ports. connected with the supply of pressure liquid, with the said one working space of the motor unit, and with a reservoir respectively, a piston valve member being slidable in said bore and having a pair of recesses; or equivalent which are in permanent register respectively with the outer two of the said ports, and the piston valve member being :formed, between the recesses *or equivalent, with a landhaving an axial width which is substantially equal to the corresponding dimension of the intermediate port, so that said land substantially closes thep'ort Whenthe piston valve member is in an intermediateposition. For convenience in manufacture the'fintermediate port may be constitutedtbyia-pluralityidf slots which are together very slightly greater'in axial width than the width of the land in the piston valve member.

The invention is illustrated by way of'exampl'e in the accompanying diagrammatic drawings, in which:

Figure '1 representsin side elevation the-relevant parts of a profile duplicating "machine, the hydraulic control system *being shown "together with the connecting pipelines Figure 2is-a'sectional elevation of the'relay valve shown in "Figurel 'the -valve being drawn toan enlarged scale;

"Figure '3 isa sectionahplan taken on the line 3-3' of Figure2;

Figure 4 is -a sectional plan 'taken on :the 'line 44 of Figure 2;

Figure 5 is a fragmentary -'seoti'onal'elevation showing in an exaggerated manner *the '-way in which the control ports of the follower valve andthe relay-valve are manufactured, the piston valve -member 'sbeing indicated in broken lines; and

Figure T6 is a sectional elevation o'f a modified construction: of "relay valve.

supporting member H, whichlatter is mounted upon the body of fthemachine (not shown) '50 that it is capable of rising and 'ifalling movement. The table In is adapted to be both Lied longitudinally :and "traversed :relatively to the .supporting member H, -the :usual longitudinal "feed wheelbeing indicated at BIZ. --A hydraulic cylinder 13 having a =double-aoting .piston 14 is also mounted upon the hody aof the ;machine, a piston rod 15 \being connected at 16 with the supporting member 41, so that the latter, in company with the table it), can -.be moved upwards 'ordownwards :by the piston :"4, said :piston normally bearing the weight of the supporting member H and the .table l0. A .pillar I'I forming part of, or attached to, the body of the machine has a horizontal portion 18, one end of which supports a casing l9 usually .containing reduction gearing for connecting the spindle of an eleotrimmotor 2'5 with .a milling cutter or like rotary tool 2| mounted upon a spindle '22. As shown, .thetool 2'l 'isopera'ting upon a work piece '23, which is clamped to "the table ID in order that 'its'pro'filemay be .made to correspond with the shape 'of fa pattern "or model 2'4, alsoclamped to "the :table #0. .The requisite variations in the thickness of the work piece '23 to correspond with the pattern or model 24 are obtained by causing the table I 0 to rise and fall as it'is-movedmanually or otherwise in 'a'ho'r'izontal plane. 'This efiectis brought about by a follower 25, which can be the same in shape and size as the sur face of revolution :of the cutter :or equivalent 21 If desired, however, the follower can be of a different shape, suitable allowance being made in the shape and/or size of the pattern or model 24. The follower 25 is attached to a holder 26 at the lower end of a rod 21 forming part of a follower valve device, which latter is indicated generally at 23 and is secured as shown to the horizontal portion [8 of the pillar IT.

The follower valve 28 comprises a body 20, into the lower part of which is screwed a tubularfibu'sh 38 arranged to carry a ball bearing 3| of the self-aligning type. The rod 21 is freely slidable through the ball bearing 3| and it's'downward :movement is limited by the engagement of a collar 32 with the ball bearing 3|, thus enabling the lowermost position of the rod21, and consequently the follower 25, to be regulated "by suitable adjustment of the bush 30. The upper part of the rod 21 is conical, as indicated at 33, and the apex is arranged to en- "gagewith a-reees's 3 4 formed in a piston valve member 35. This is 'slidably mounted within "a sleev e 35 fitted "tightly into "the body 29, :said

piston valve member being :formed 'with -a pair of circumferential grooves 31 and 58, 'leaving between them a land 39. When the pistonvalve member 35 is in'its normal position vof equilibrlum this land closes, or substantially closes, a port or set of ports 40 formed -in the sleeve -36, said port or ports being arranged to be in permanent register with a groove '4! formed in the bore of the bodyZS. Otherports'42 and-43 in the sleeve 36 are arranged to be in permanent communication with the grooves 31 'arid58 'respectively. The piston valvemember 35 isimaintained in engagement with the conicalmember -33 bymeans of 'a-coiled compression spring M hearing at its upperend against an abutment 45 which is adjustable by meansof abolt -65.

The system also includes a relay valve, which isiindicated at 41 in Figure lyand is for convenience disposed within acontainer 48 serving as a reservoir :for spare working liquid. The

construction of the relay valve is shown more clearly in Figures 2, 3 and 4. It comprises a body 49 having-a tubular sleeve 50 within which .a piston valve-member 5i fits slidably, circum- =ferential grooves 52 and 53 in said piston valve member 5| being in permanent communication with connections 54 and 55 by means of ports 56 and 51 respectively in the sleeve. Between the grooves 52 and 53 the piston valve member 5| has a circumferentially continuous land '58, which co-operates with a series of three slots 59 arranged circumferentially, as will 'be seen in Figure 3, said slots being in permanent communication with a connection 60 owing -to theprovision ofa groove iii in the body 49. To theupper part of the body 49 a tubular spring housing'62 is secured, this being formed with a tubular spigot 53 which is internally screw- "threaded to receive an adjusting screw 64, while the other side of the spigot 53 carries a nut'65 "serving to clamp the relay valve 4'! on tothelid 66 of the reservoir 48. A cap '61 serves to pro- The lower end "of ing against the upper end of a coiled compression spring 69, while the lower end of this spring bears downwards against the piston'valve member 5!, upon'which a locating spigot T0 is provided. The piston valve member 5| needs to be urged upwardly by pressure liquid supplied to apair of connections "H and 1'2 with out, however, placing said connections into Tcommunicationfwith one another. This is effected by providing inthe lower part of the body 49 two pairs of diametrically opposed bores, all four bores being arranged equi-distant from and parallel to the axis of the piston valve member The bores constituting one pair are indicated at 13 and 14, while the other pair is shown at 15 and 1B in Figure 4. The bores are all closed at their lower ends, the pair 13, 14 being in communication with the connection 1| by a passage 11, while a similar passage 18 joins the bores 15, 16 to the connection 12. The bores 13-16 are fitted with corresponding plungers, three of which are shown at 19, 80 and 8| in Figure 2, while the fourth, which is disposed within the bore 16, will be referred to as 82. Each is provided with a depending spigot 83, which limits its downward movement and thus prevents the corresponding passage 11 or 18 from becoming blocked. It will thus be appreciated'that when fluid pressure is present in either or both of the connections H and 12 the corresponding pair or pairs of plungers press upwardly upon the piston valve member 5|, so opposing the spring 69 with a force which is proportional to the sum of the said fluid pressures.

The lid 66 of the reservoir 48 also supports an electric motor 84 driving a liquid pressure pump 85. The latter draws in liquid through an inlet pipe 86 and delivers said liquid under pressure through a pipe 8'! to a pressure relief valve of the usual construction, indicated at 88. From thence the pressure liquid is taken through a pipe 89 which leads to the connection 54 of the relay valve 41 and also to the port 42 of the follower valve 28. The reservoir 48 also contains a drain pipe'90, which is joined to the connection 55 of therelay valve and which leads out of the reservoir to the port 43 of the follower valve. The groove 4| of the follower valve is connected by a pipe 9| leading through a control valve 92 to a pipe 93 connected with the lower working space 94 of the motor cylinder unit l3, l4; the pipe 93 also leads to the connection 1| of the relay valve 41. From the connection 12 of said valve a pipe 95 leads to the connection 60 and also to the upper working space 96 within the motor cylinder |3.

The control valve 92 comprises a body'91 having a bore 98 containing a piston valve member 99 operated by an external sliding handle I00. When the control valve is in the position shown, 1. e., its normal working position, a circumferential groove |0| connects the pipe 93 with the pipe 9| as above mentioned, a branch 99a leading from the pipe 89 being closed by the left-hand end of the valve member 99; the action of moving the handle I00 to the left causes the pipe 9| to be stopped and connects the pipe 93 with thepipe 89, thus enabling pressure liquid from the supply to be fed directly to the lower working space 94 of the motor cylinder |3 for adjusting the height of the table l0.

It has been found in the case of the follower valve and also the relay valve that it is advantageous to allow a very slight, but nevertheless appreciable, leakage to take place past the central land 39 or 58, and for this purpose the slots 40 or 59 formed in the sleeve 36 or 50 are preferably arranged in the manner shown in Figure 5. The slots, which are three in number, are made as nearly as possible the same width as the central land 39 or 58, but each is cut in a very slightly oblique or helical direction so that the adjoining ends of each adjacent pair of slots are staggered by a distance A, as shown to a greatly exaggerated extent in Figure 5. In actual practice the distance A would be of the order of 0.001 inch, thus in effect making the extreme width of the slots, taken as a whole, approximately 0.001 inch greater than the width of the land 39 or 58. The obliquity of the slots gives, however, a gradual cut-off asthe land moves away from a dead central position, thus stabilising the valve member and avoiding a tendency to chatter or vibrate.

[The action of the hydraulic control system is as follows. The follower valve 28, which receives pressure liquid from the supply pump through the pipe 89, controls the admission of said liquid to the working space 94 below the piston l4, the liquid in said working space normally being maintained at apressure sufficient to sustain the weight of the table l0 and the parts associated therewith, as well as the downward force exerted by the pressure liquid in the upper working space 96. As the table In is moved in a horizontal plane, for example by means of the feed wheel l2, the follower 25 scans the surface of the pattern or model 24, with which it remains in contact, and consequently tends to rise and fall in sympathy with the variations in the surface of said pattern or model. For instance, if the follower 25 tends to rise, due to the increasing thickness of the pattern or model 24, the rod 21 raises the piston valve member 35, thus enabling the lower circumferential groove 38 of the follower valve to connect the pipe 9| (and consequently the low er working space 94) with the drain pipe this, of course, reduces the pressure of the liquid in the lower working space 94 of the motor cylinder 3-'-and therefore allows the piston I4 to drop. The downward movement of the piston l4, and consequently the table I9, is assisted, however, by'the action of the relay valve 41, which at the same time increases the pressure present in the upper working space 96. This action is brought about as follows. The piston valve member 5| is normally held in its central position, as shown, by the combined action of the four plungers 19-02 pressing upwards in oppositionto the spring 69. When, therefore. the liquid pressure in the lower working space 94 is reduced, the-upward thrust upon the plungers 19 and 80 is correspondingly lessened and the spring 69 moves the piston valve member 5| in a downward direction. This comiects the upper working space 96 of the motor cylinder I3 with the supply of pressure liquid by way of the pipe 95, the slots 59, the circumferential groove 52 and the port 56, so that the pressure in said upper working space 96 is increased. The increase is, however, limited, owing to the fact that the upward. force exerted by the plungers 8| and 82 in the relay valve 41 correspondingly increases, and therefore, when the total upward thrust ex 'erted by the four plungers 19-82 regains its original value, the piston valve member 5| is restored toits'central or off position. From this it follows that-if the plungers 19-82 are all eq i diameter, the increase of liquid pressure in the upper working space 96 is equal to the reduction of liquid pressure in the lower working space 94.

The lowering of the table l0, which is thus brought about hydraulically by the combined action of the follower valve 28 and the relay valve 41, restores the follower 25 to its original position level with the cutter 2|, and the piston valve member 35 of the follower valve is thus returned :to itsintermediate or "off" position. As a consequence the pressure of the liquid in-the lower working space '94 regains its original value, so that the total upward thrust of the plungers '19-'82 becomes greater than "normal; the piston valve member 5| is thus lifted slightly and the excess .pressure within the upper working space 96 of the cylinder I3 allowed to escape through the connection 55 and into the reservoir.

The action which occurs when the follower 25 is allowed to fall due to a decrease in the thickness of the pattern or model 24 is the opposite to the above. The spring 44 moves the piston valve member 35 of the .iollower valve 28 downwardly, thus connecting the lower working space 94 of the motor cylinder I3 with the supply of pressure liquid in the pipe 90 and port 43. This increases the pressure within said lower working space 94 and at the same time causes the upward thrust of the plungers "I9 and 3!) in the relay valve 4'! to be increased, so that the piston valve 51 is moved upwards and connects the upper working space 95 of the motor unit I3 with the interior of the reservoir '48 by way of the connection 55. As a result the pressure in the upper working space 93 falls until the piston valve member 5| of the relay valve is restored. to its off position. The relay valve in this way serves to control the value of the sum of the liquid pressures in the two working spaces 94 and 96 of the motor cylinder I3, so that when one of these pressures is reduced the other is automatically increased to a corresponding extent and vice versa; this, of course, renders the piston Ill very responsive to the movements of the follower 25 and has the effect of improving the accuracy of-the work pro duced by the cutter or equivalent 2 I.

A modified form'of relay valve is illustrated diagrammatically in Figure 6, and comprises a body I02 having two cylindrical coaxial bores I03 and I04 forming a pair of chambers separated by a .partition I 05. A thrust rod I06 passes through a partition I05 in a liquid-tight manner and cooperates at one end with a piston I'I slidable within the bore I04, said piston being urged towards the left by a coiled compression spring I08 bearing against an abutment I09 which is adjustable by means of a screw I II]. The other end of the thrust rod I06 has a plate III against which bears a compression spring II2, which serves to control the .position of a piston valve member I similar in construction to the one shown in Figure 2. This piston valve member 5!, which has a pair of circumferential grooves 52 and 53, with a land 58 between them, is urged towards the right by a coiled compression spring I I3, and is arranged to control the fiow of liquid between three ports '56, 57 and '59 axially spaced along the bore I03. The spring H3 is relatively weak compared with the two springs I08 and H2, which latter act in combination to urge the piston valve member 5| towards the left. The left-hand end of the bore I03 constitutes a work- 'ing space II 4, which is connected by a pipe 95 with the port 59 and also with the upper working space 96 of the motor unit (as in Figure 1);

the ports 56 and v5'! are connected with the pressure supply and with the reservoir respectively by a pipe 89 and 90; and the working space II5 between the piston I01 and the partition I05 is connected with the lower working space 94 of the motor unit by means of a pipe 93.

It will be seen that the liquid pressure within the working space H4 in effect opposes the force ofboth of the compression springs H2 and I08, and that the liquid pressure in the working'space 5 acts in opposition to the spring I08.- Asa consequence an increase in pressure in the lower working space 94 of the motor unit causes the spring I08 to be counteracted to a corresponding extent, thus shifting the piston I01 towards the right and enabling the piston valve member 5| to move also towards the right. This tends to place the upper working space 96 of the motor unit into communication with the reservoir 48 of the system, and thus reduces the pressure in the said upper working space 95. As the pressure in the upper working space 96 decreases, so does the pressure in the working space II4 of the relay valve, so that the piston valve member 5| is returned "to its centralised or ofi position. Similarly a reduction in the pressure of the working liquid in the lower workingspace of the motor unit is accompanied by a corresponding reduction in the working space N5 of the relay valve, thus enabling the spring I08 to expand and move the piston valve member 5| to the left. This places the upper working space 96 of the motor unit into communication with the pressure supply, thus raising the pressure in the upper Working space 96 of the motor unit and also the space I.I4-of the relay valve until the piston valve member-5| is returned to its centralised position.

It will be appreciated that the system whichhas been described is given by way of example and that various modifications are possible to suit requirements. Thus any suitable means may :be used for operating the follower valve, and in one possible arrangement a bell-crank lever connects the follower with the valve member, adjustable stops being provided, if desired, to'limit the movement of the valve member. Also the relay valve may take other forms, and it maybe desirable in some cases to counterbalance the table I0 (or the tool and follower head I 8, I9, 28, where this is arranged to move, the table I0 remaining stationary), thus reducing the load which is carried by the motor piston I4.

What we claim is:

1. In a duplicating machine including a pat- :tern follower, a cutting tool and a work support,

means for relatively moving the cutting tool and work support toward and away from one another in response to pattern controlled movement of the follower which comprises, a source of pressure fluid, a fluid reservoir; a double acting piston and cylinder unit having two working spaces and having its movable member connected to relatively move the work support and the cutting tool toward and away from one another, a valve housing, a conduit connecting said housing with one of the working spaces of the piston and cylinder unit, a conduit connecting said housing with said source, a conduit connecting said housing with said reservoir, valve means in said housing connected to move in response to displacement of the follower, from a neutral position closing the first named conduit, in one direction to establish communication between the source and the first named conduit and in the other direction to establish communication between the reservoir and the first named conduit, and a valve member which is operable separately from said valve means in response to fluctuation in pressure from a predetermined value in the first named conduit for establishing a passageway between the other working space of said piston and cylinder unit and said source upon decrease in pressure in said first named conduit and for establishinga passageway between said otherworking space of said piston and cylinder unit and said reservoir upon increase in pressure in said first named conduit.

2. In a duplicating machine including a pattern follower, a cutting tool and a work support, means for relatively moving the cutting tool and work support toward and away from one another in response to pattern controlled movement of the follower which comprises, a source of pressure fluid, a fluid reservoir, a double acting piston and cylinder unit having two working spaces and having its movable member connected to relatively move the work support and the cutting tool toward and away from one another, a valve housing, a conduit connecting said housing with one of the working spaces of the piston and cylinder unit, a conduit connecting said housing with said source, a' conduit connecting said housing with said reservoir, valve mean in said housing connected to move in response to displacement of the follower, from a neutral position closing the first named conduit, in one direction to establish communication between the source and the first named conduit and in the other direction to establish communication between the reservoir and the first named conduit, a valve body, conduits connecting said source and reservoir with the other WOrklng space of said piston and cylinder unit through said body, a piston valve in said body movable, from a normal position closing the conduits from said source and reservoir to said other working space;'in one direction to establish connection between said source and said other working space and in the other direction to establish connection between said reservoir and said other working space, a conduit connecting one end of said body to a pressure zone common to that of the first named conduit so that the piston valve is urged in one direction by the pressure therein, and a spring acting on the other end of said piston valve in opposition to the fluid pressure, whereby upon a reduction in fluid pressure in said first named conduit below a predetermined value the spring will move the piston valve to establish connection between said source and said other working space while upon an increas in pressure in said first named conduit above said predetermined value said piston valve will be ppositely moved by the pressure fluid to establish connection between said reservoir and said other working space.

3. In a duplicating machine including a pattern follower, a cutting tool and a work support, means for relatively moving the cutting tool and Work support toward and away from one another in response to pattern controlled movement of the follower which comprises, a source of pressure fluid, a fluid reservoir, a double acting piston and cylinder unit having two working spaces and having its movable member connected to relatively move the work support and the cutting tool toward and away from one another, a valve housing, a conduit connecting said housing with one of the working spaces of the piston and cylinder unit, a conduit connecting said housing with said source, a conduit connecting said housing with said reservoir, valve means in said housing connected to move in response to displacement of the follower, from a neutral position closing the first named conduit, in one direction to establish communication between the source and the first named conduit and in the other direction to establish communication between the reservoir and the first named conduit, a. valve body, conduits connecting said source and reservoir with the other working space of said piston and cylinder unit through said body, a piston valve in said body movable, from a normal position closing the conduits from said source and reservoir to said other working space, in one direction to establish connection between said source and said other working space and in the other direction to establish connection between said reservoir and said other working space, one end of said valve body defining a plurality of like axially extending cylinders, a plurality of plungers connected to said piston valve and slidable in the respective cylinders of the valve body, -a conduit connecting a pressure zone common to that of the first named conduit with some of the cylinders in said valve body, a conduit connecting a like number of the cylinders in said valve body withthe other working space of said piston and cylinder unit, and a spring acting on the end of said piston valve remote from the end provided with plungers in opposition to said plungers whereby said other working space will be connected with either the source or reservoir depending upon whether the fluid pressure acting on the plungers overcomes or is overcome by the action of the spring upon fluctuation in pressure in the first named conduit from a predetermined value.

4. In a duplicating machine including a pattern follower, a cutting tool and a work support, means for relatively moving the cutting tool and work support toward and away from one another in response to pattern controlled movement of the follower which comprises, a source of pressure fluid, a fluid reservoir, a double acting piston and cylinder unit having two working spaces and having its movable member connected to relatively move the work support and the cutting tool toward and away from one another, a valve housing, a conduit connecting said housing with one of the working spaces of the piston and cylinder unit, a conduit connecting said housing with said source, a conduit connecting said housing with said reservoir, valve means in said housing connected to move in response to displacement of the follower, from a neutral position closing the first named conduit, in one direction to establish communication between the source and the first named conduit and in the other direction to establish communication between the reservoir and the first named conduit, a valve body defining coaxial, main and auxiliary chambers, a piston valve member in said main chamber, a piston in said auxiliary chamber having a thrust rod extending through a partition into the main chamber, a pair of compression springs disposed within the respective chambers and acting to urge the piston in opposite directions and cumulatively on the piston valve to urge it in one direction, a relatively weak spring acting on said piston valve in opposition to said pair, a conduit connecting said first named conduit to said auxiliary chamber so that the fluid under pressure in the first named conduit opposes the thrust of the spring in said auxiliary chamber, a conduit connecting the end of said main chamber to the other working space of the piston and cylinder unit so that the piston valve is urged against the thrust of said pair of springs by the pressure in said other working space, and conduits connecting said source and said reservoir to said other working space of the piston and cylinder unit through said auxiliary chamber and piston valve, said piston valve establishing communication between said source and said other working space upon displacement in one direction away from a normal position and establishing communication between said; reservoir, and said workingspace upon dis! placement in the other direction, the direction of displacement depending upon whether thereis upward or downward fluctuation away from a predetermined pressure value in said auxiliary chamber.

5. A duplicating machine as claimed in claim 2 in; which the valve body is provided with a central axial bore.- and has; three axially aced por s therein connected; withthe; conduit leading to the source, with the conduit leading to, the other working space of the piston and cylin r; un and with the conduit leading to the reservoir respectively, and in which the piston valve, is providedv with a pair of axially ced annular ..e cesses each in permanent reg s ry wit t e cor responding outer port, said piston valve member having a land of axial width substantiazlyequal to the corresponding dimension of the intermedi-. ate port, said land being located between the re cesses so as to close the intermediate port when the piston valve is in normal position.

6. A profile duplicating machine as claimed in claim 2 in which the valve body is provi ed with a central axial bore and, has three axially spaced ports therein connected with the conduit leading to the source, with the conduit leading to the other working space of; the. piston and cylinder unit. and with the conduit leading to said, reser-- voir,.respectiye1y. and. in which the piston valve provided with a pair of axially spaced annular recesses each in registry with. the corresponding outer port, said piston valve member having a land of an axial width substantially equal to, the corresponding dimension of the intermediate port, said land being located between the recesses so as to substantially close the intermediate, port when the piston valve is in neutral position, said intermediate port;- consisting of, a lurality of slots which are together very slightly greater in axial width than the land on the; piston. member.

7. A profile duplicating machine as claimed in claim 1 in; which the valve housing is provided with an axially extending central bore and three axiallyspaced ports connected with the conduit leading to the source, the conduit leading to one side of the piston and cylinder unit, and the con duit leading; to said; reservoir, respectively, and in which the valve, means consists. of a piston member slidable in the bore of the housing, said piston member being provided with a pair of axially spaced annular recesses. each in registry with the co responding outer port, said piston member having a land of an axial width substan-. tially equal to. the corresponding dimension of the intermediate port, located between the recesses so as to. substantially close the intermediate port when the piston valve. is in neutral position.

EDWARD CLAUDE SHAKESPEARE CLENCI-I. QUINTIN HEALEY CARLTON. 

